This invention relates generally to the art of gas generation and more particularly to compositions and methods for generating hydrogen gas.
There are currently only a limited number of methods available to produce hydrogen on a small scale. For example, hydrogen gas is generated by the reaction of metals or metal hydrides with acids, bases, water, alcohols, etc. Hydrogen gas can also be stored in and released from pressurized gas cylinders. However, these methods are not applicable when severe weight and/or volume restrictions are imposed on a system which must generate relatively small amounts of hydrogen gas (up to about 250 liters) in a short time (less than one minute). The following example will illustrate this point: The exothermic reaction of LiH+H.sub.2 O.fwdarw.LiOH+H.sub.2 with 27 kcal/mole hydrogen. Assuming 100 percent completion of the reaction without an available external heat sink, then well over 300 grams of water would be needed to produce 1 mole of hydrogen gas (i.e., 18 grams of water as reactant and the remainder for a heat sink) in order to prevent boiling of the water and the formation of a hydrogen/steam mixture. This means that the weight/volume ratio (reactants (grams) per liter of hydrogen generated) is greater than 14 grams per liter.
Another reaction which has been proposed to generate hydrogen is based on the thermal decomposition of hydrazine bisborane. This reaction is represented by the following equation: ##STR1## Although in theory this reaction is very favorable in terms of hydrogen produced per gram of reactant (weight-volume ratio is 0.54 gram per liter assuming 100 percent yield), the temperature required to keep the reaction going is high enough to melt glass fiber cloth and the hydrogen produced is at its ignition temperature. In addition, hydrazine bisborane is not commercially available and it is difficult to handle because of its instability.
U.S. Pat. Nos. 3,734,863, 3,862,052, 3,931,395, 3,977,990, and 4,022,705 to Beckert et al., which are incorporated herein by reference, teach various hydrogen generating compositions and their methods of preparation. More specifically, U.S. Pat. Nos. 3,734,863, 3,862,052, and 3,931,395 disclose reacting ammonium or hydrazinium salts with suitable complex metal hydrides as expressed by the following general formulas and general equations: ##EQU1## where x is an acid group such as an inorganic acid group like halogen (Cl,Br,F), sulfate (SO.sub.4), and the like, n is the valency of the acid group, Y is a mono- or divalent metal capable of forming complex hydrides, such as alkali and alkaline earth metals like Li, Na, K, Mg, Ba, Ca, etc., m is the valency of said metal and Z is a trivalent metal capable of forming complex hydrides, such as B, Al, and the like. Similarly, U.S. Pat. No. 3,977,990 to Beckert et al. teaches that the hydrogen gas evolution rates and the gas temperatures of certain hydrogen gas generating compositions are modified by adding compounds such as LiAlH.sub.4 which thermally decompose in the reaction zone producing hydrogen while lowering the reaction temperature; and certain acetonates, certain metal oxides, and the like which, when added in relatively small amounts accelerate the hydrogen gas evolution rate.
While these methods and compositions are satisfactory in providing hydrogen at a fast rate from solid, storable compositions, complex metal hydrides are commercially available only to a limited extent, and they are relatively expensive. Especially for large-scale uses such as in laser or fuel cell applications it is highly desirable to use compositions that are less costly.